Magnesium sulfate monohydrate (MgSO.sub.4.H.sub.2 O) and the higher hydrates of magnesium sulfate are naturally occurring materials having many uses. For example, they are known as fireproofing materials, as catalyst carriers, for use in warp-sizing and loading cotton goods, weighting silk, dyeing and calico printing, as an ingredient in mineral waters, and are used in ceramics, fertilizers, paper, cosmetic lotions, dietary supplements, laxatives, and as a therapeutic bath salt.
A number of methods are known in the prior art for the production and purification of magnesium sulfate (MgSO.sub.4) and its hydrates. For example, these materials are sometimes produced by reacting sulfuric acid (H.sub.2 SO.sub.4) with either magnesia (MgO) or magnesite (MgCO.sub.3). However, this method is very expensive and the magnesium sulfate produced may still require subsequent purification due to impurities present in the magnesia or magnesite feed stock.
Alternatively, hydrated magnesium sulfate may be produced by dissolving kieserite (magnesium sulfate monohydrate) and then subsequently refrigerating this resulting solution.
Kieserite is mined in large quantities, for example in Germany, and after washing, supplies most of the world's magnesium sulfate market. It contains various impurities, however, including halite (i.e., sodium chloride) and, in addition, it comprises only 89-93% magnesium sulfate monohydrate. Moreover, since the mined kieserite is only partly soluble in ambient water it is not acceptable for use in certain applications, for example, as a therapeutic bath salt or as a laxative.
The refrigeration process described above requires the refrigeration of an unduly large amount of magnesium sulfate-containing solution to obtain a commercially desirable quantity of epsomite and is thus not particularly commercially feasible. For a further discussion of the refrigeration process, see, e.g., Fernandez-Lozano, "Recovery of Epsomite and Sylvite from Seawater Bittern by Crystallization", Fourth Symposium on Salt, Alan H. Coogan, The Northern Ohio Geological Society, Inc., Cleveland, Ohio, Vol. 2, pp. 501-510 (1974).
Epsomite is a heptahydrate of magnesium sulfate having the chemical formula MgSO.sub.4.7H.sub.2 O. It is often found mixed with halite (i.e., NaCl) in nature. Such mixtures are readily isolated from seawater by solar evaporation. In the past, however, these mixtures have not been relied upon as a source of magnesium sulfate or its hydrates since epsomite and halite react upon contact in certain aqueous solutions to form double salts, i.e., a condition wherein two salts crystallize from a melt or from a solution in simple stoichiometric proportions, forming a distinct crystal lattice. The double salts formed in this case are mainly of magnesium and sodium sulfate. An example of such a double salt is astrakanite (MgSO.sub.4.Na.sub.2 SO.sub.4.4H.sub.2 O), which is formed according to the following reaction: EQU 2MgSO.sub.4.7H.sub.2 O+2NaCl.fwdarw.MgSO.sub.4.Na.sub.2 SO.sub.4.4H.sub.2 O+MgCl.sub.2.
The formation of such double salts, when they occur, makes the extraction of magnesium sulfate from the mixture extremely difficult.
Consequently, the separation of epsomite and halite must take place in a medium in which the two substances cannot react, i.e., so as not to form double salts. A simple, cost-effective method has thus long been desired for separating these two materials in a manner to provide substantially pure magnesium sulfate products without the formation of double salts as described above.